In recent years, higher output densities and improved energy densities have been required of electrochemical devices including cells and capacitors. Organic electrolytic solutions have found wider use than aqueous electrolytic solutions from the viewpoint of voltage resistance. Examples of organic electrolytic solutions are those prepared by dissolving alkali metal salts or solid ammonium salts in an organic solvent such as propylene carbonate. Electrolytic solutions of the former type are used for lithium ion cells, while those of the latter type are used for electric double-layer capacitors. Organic electrolytic solutions are inferior to aqueous solutions in electrical conductivity, and numerous studies have been made on organic solvents or electrolytes to obtain improved electrical conductivity. As a result, Patent literature 1 (JP 1991-58526 A) discloses asymmetric ammonium salts for use as electrolytes for electric double-layer capacitors. Ue et al., J. Electrochem. Soc. 141(2989) 1994 shows detailed investigations into kinds of tetraalkylammonium salts and electrical conductivity thereof. Tetraethylammonium tetrafluoroborate and triethylmethylammonium tetrafluoroborate are generally in use.
The electrical conductivity of nonaqueous electrolytic solutions comprising such a solid electrolyte dissolved in a solvent varies with the concentration of the electrolyte. With a rise in the concentration, the ion concentration of the solution increases to increase the electrical conductivity, which will reach a maximum in due course. The electrical conductivity reaching the maximum starts to decrease presumably because the electrolyte becomes difficult to dissociate and increases in viscosity at the same time owing to increased interaction between the solvent and ions and between the ions as the number of ions increases in the electrolytic solution. When further increasing in concentration, the electrolyte becomes no longer dissociable, and the concentration of the electrolyte levels off. Thus, an attempt to increase the concentration of the electrolyte encounters the problem that the electrolyte becomes less soluble. Another problem is also experienced in that when electrolytic solutions having an electrolyte dissolved therein at a high concentration is used in an environment of low temperature, a salt will separate out to impair the electrical conductivity of the solution. Solvents of high dielectric constant are usually preferred for dissociating electrolytes to a higher degree, and propylene carbonate, ethylene carbonate, gamma-butyrolactone, etc. are in use. Suitable to use as electrolytes are tetraethylammonium tetrafluoroborate, triethylmethylammonium tetrafluoroborate and the like which are relatively soluble in solvents of high dielectric constant, whereas these electrolytes are limited in solubility to a concentration of about 2 M at room temperature and have the disadvantage of permitting separation of crystals when to be dissolved to higher concentrations or at lower temperatures. These electrolytes are almost insoluble in solvents of low dielectric constant, failing to form electrolytic solutions which are useful as such.
When propylene carbonate, ethylene carbonate, gamma-butyrolactone or the like is used as the solvent for applications necessitating a high voltage, the electrolyte is governed by the solvent decomposition voltage even if the electrolyte has high voltage resistance, with the result that the conventional capacitors are limited to about 2.5 V in operating voltage if highest. If the capacitor is operated at voltage exceeding 2.5 V, the electrolytic solution (mainly the solvent) undergoes electrochemical decomposition, becomes seriously impaired in performance and produces undesirable phenomena such as evolution of gas. In the application of capacitors as energy storage devices to mobile bodies such as hybrid cars and electric motor vehicles, improved energy capacities are demanded, and a higher operating voltage is effective means for giving an improved energy density, whereas it has been impossible to improve the voltage resistance with use of conventional electrolytic solutions, hence a need for electrolytes and solvents of higher voltage resistance. Although chain carbonate solvents are solvents of higher voltage resistance, conventional electrolytes such as tetraethylammonium tetrafluoroborate and triethylmethylammonium tetrafluoroborate are low in solubility in these solvents which are low in dielectric constant, and are not usable as electrolytic solutions.
Found in recent years are salts having a melting point around room temperature or salts having a melting point not higher than room temperature (salts melting at room temperature). It is known that even if solid at room temperature, such salts dissolve in organic solvents at a higher concentration than usual electrolytes. Furthermore, the salts melting at room temperature are miscible with a specific organic solvent in a desired ratio. Accordingly, these salts afford electrolytic solutions having a high concentration not available by dissolving conventional solid electrolytes in organic solvents, while although having a high concentration, the solution is less likely to encounter the problem that the salt will separate out in a low-temperature environment. The salt melting at room temperature is itself liquid and is therefore usable singly as an electrolyte.
It is also known that salts melting at room temperature, although liquid, are low in vapor pressure and not easily combustible because they comprise ions only. Accordingly, when dissolved in an organic solvent at a high concentration, the salt melting at room temperature serves as a flame retardant for electrolytic solutions.
Typical of such salts melting at room temperature is 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI·BF4). The salt EMI·BF4 has a high electrical conductivity, and the application of this salt to electrochemical devices including lithium secondary cells and electric double-layer capacitors is under study. However, the imidazolium salt is about 4 V in electrochemical stability, such that when applied to electric double-layer capacitors, the salt is about 2.5 V in the upper limit of operating voltage and is still in limited use.
Research has been made in recent years on salts melting at room temperature and stable in a wider potential range. For example, salts melting at room temperature and comprising a cationic component with an aliphatic ammonium skeleton as disclosed in Patent Literature 2 (Japanese Patent No. 2981545) are at least 5.8 V in voltage resistance and are considered to be applicable to lithium secondary cells. The salts melting at room temperature and having an aliphatic ammonium skeleton in the cationic component nevertheless have the drawback of being generally high in viscosity and low in electrical conductivity. Although improved in electrical conductivity when mixed with an organic solvent, the conductivity level is still lower than that of the solutions of conventional solid electrolytes in organic solvents.
Patent Literature 3 (WO 02/076924) discloses that aliphatic ammonium salts having an alkoxyalkyl group introduced thereinto are highly soluble in a nonaqueous organic solvent and are less likely to separate out at low temperatures, while electrolytes still higher in solubility in organic solvents, voltage resistance and electrical conductivity are demanded.
Even in the case where the salts melting at room temperature, having diethylmethylmethoxyethylammonium as a cation component and disclosed in Patent Literature 3 are dissolved in an organic solvent, the solutions are lower in electrical conductivity than the electrolytic solutions prepared by dissolving conventional solid electrolytes (e.g., triethylmethylammonium tetrafluoroborate, etc.) in an organic solvent. The disclosed salts still remain to be improved in solubility in a chain carbonate, and electrolytes are demanded which are higher in solubility in organic solvents, voltage resistance and electrical conductivity.
An object of the present invention is to provide a quaternary ammonium salt which is high in electrical conductivity and voltage resistance.
Another object of the invention is to provide an electrolyte which is high in solubility in organic solvents, voltage resistance and electrical conductivity.
Another object of the invention is to provide an electrolytic solution which is high in voltage resistance and electrical conductivity.
Another object of the invention is to provide an electrolyte which affords an electrolytic solution of high electrolyte concentration when dissolved in a solvent and consequently provide an electrochemical device usable at a high voltage and having a high discharge capacity and great current discharge performance.